fix: handle long fractional Unix timestamps

[immanuwell]

Tiny parser fix. Before this, a Unix timestamp with a long fractional part got validated before truncation, so it could overflow int64 and fail. Kinda annoying, but real: logs --since/--until feed user timestamp strings through this path.

Repro before the fix:

go test ./pkg/timestamp

with 1136073600.123456789123456789123, it errors with value out of range instead of truncating to 9 ns digits.

Now the parser truncates first, then parses. Existing short fractions still work.

Checked:

go test ./pkg/timestamp
go test ./pkg/...
git diff --check origin/main..HEAD